In a heat-developable light-sensitive material, it is desirable to use a base in order to accelerate development by heat, and to increase the stability of the light-sensitive material, it is necessary to use the base in the form of a precursor. In practice, such base precursors are required to satisfy both requirements of high stability at ordinary temperature (e.g., 20.degree. C.) and rapid decomposability at the time of heating.
Base precursors which have heretofore been known include ureas as described in U.S. Pat. No. 2,732,299 and Belgian Pat. No. 625,554, ammonium salts of urea or urea and weak acids as described in Japanese Patent Publication No. 1699/65, hexamethylenetetramine and semicarbazide as described in U.S. Pat. No. 3,157,503, triazine compounds and carboxylic acids as described in U.S. Pat. No. 3,493,374, dicyandiamide derivatives as described in U.S. Pat. No. 3,271,155, N-sulfonylureas as described in U.S. Pat. No. 3,420,665, amineimides as described in Research Disclosure, RD No. 15776 (1977), and salts of heat-decomposable acids such as trichloroacetic acid as described in British Pat. No. 998,949.
However, image-forming materials containing such base precursors have serious disadvantages. One of the disadvantages is that the base precursors fail to satisfy the above requirements of high stability during storage at ordinary temperatures and rapid decomposition during the process of development. Therefore, a high image density cannot be obtained, or the base is released during the storage, leading to a serious decrease in the density/fog ratio of the image.
In order to overcome the above problem, Japanese Patent Application (OPI) No. 168441/84 (the term "OPI" as used means a "published unexamined Japanese patent application") discloses sulfonylacetic acid salts, and Japanese Patent Application No. 55700/83, propiol acid salts. These base precursors are excellent in that a high density image can be obtained in a short period of time. With respect to the stability during the storage, however, they are not sufficiently satisfactory. In particular, they have a disadvantage in that when light-sensitive materials containing them are stored at high temperatures, the formation of fog is significant and desensitization is large.